CO2 poses threat for Monarch butterflies

Mounting levels of atmospheric carbon dioxide pose a previously unrecognised threat to monarch butterflies, by reducing the medicinal properties of milkweed plants that protect the iconic insects from disease, a study has found.

Milkweed leaves contain bitter toxins that help monarchs ward off predators and parasites, and the plant is the sole food of monarch caterpillars. Researchers at the University of Michigan in the US grew four milkweed species with varying levels of those protective compounds, which are called cardenolides.

Half the plants were grown under normal carbon dioxide (CO2)levels, and half of them were bathed, from dawn to dusk, in nearly twice that amount. Then the plants were fed to hundreds of monarch caterpillars.

The study showed that the most protective of the four milkweed species lost its medicinal properties when grown under elevated CO2, resulting in a steep decline in the monarch’s ability to tolerate a common parasite, as well as a lifespan reduction of one week. The researchers looked solely at how elevated carbon dioxide levels alter plant chemistry and how those changes, in turn, affect interactions between monarchs and their parasites.

“We discovered a previously unrecognised, indirect mechanism by which ongoing environmental change — in this case, rising levels of atmospheric CO2 — can act on disease in monarch butterflies,” said Leslie Decker, first author of the study published in the journal Ecology Letters.